Events - inversion of control methods - javascript

How does listenTo / stopListening work with respect to on / off?
In the console, I'm experimenting with the Backbone event system as such ...
// works
Backbone.on('x', function(){console.log('x happened');})
// works
Backbone.trigger('x');
// works
Backbone.off('x');
// works
Backbone.once('x', function(){console.log('x happened');})
I'm trying to extend my example to use listenTo() and stopListening().
These are listed as the inversion of control types. Is there a simple way to show there use as above?
The primary difference, as viewed from the source, is that the first parameter needs to be an object.

It is exactly the same, except it's an inversion of control: listenTo binds the events on the listening object, rather than the triggering object. This is most useful for cleaning up View event handlers, because the view now knows which events it's listening to and can unbind them when it's removed. With the original system, only the triggering object (ie, the model) would have direct knowledge of the bound events.
There's a good explanation of the concept here. It shows how people used to do it before it was added to Backbone.
In your example, you don't really have a "listening" object, since your handler is just an anonymous function. But it would be something like obj.listenTo(Backbone, "x", obj.alert);, where alert would be a handler method on obj.

Because listenTo and stopListening work on a different object you need to create another object that has access to the event system. One easy way is to create a view object as such.
var View1 = new Backbone.View();
Next setup a listener:
Backbone.listenTo(View1, 'x', function(){console.log('I heard x');});
Now trigger the event on View1
View1.trigger('x');
Finally remove the listener
Backbone.stopListening(View1,'x');
That covers the 6 main methods of the event system.
Tested and working...

Related

Simple event system using JavaScript (ES5) - JS Custom Event

Instructions:
make this code work without modifying snippet it in any way.
Use only plain old JavaScript and no third-party libraries.
Write new code that enables the code below to work properly.
Hint: Feel free to extend native objects... even though it's typically a bad practice.
// Start with an object, any object
var myObject = {};
// Register an event on your object using
// an `on` method
myObject.on('myEvent', function(data) {
// Log the data passed to the callback
console.log(data);
});
// Trigger the event using a `trigger` method.
// Include some data when you trigger the event.
myObject.trigger('myEvent', {
company: 'ABC Corp',
location: 'WTC Bangalore, IN',
website: 'http://abc.co'
});
// Register a different event
myObject.on('yourEvent', function() {
console.log('yourEvent fired');
});
// Trigger the new event
myObject.trigger('yourEvent');
// Trigger all existing events using a special
// "star" identifier.
myObject.trigger('*');
// Remove one event by name
myObject.off('myEvent');
// Since we've removed the event, this should
// do nothing
myObject.trigger('myEvent');
// Remove all existing events
myObject.off();
// Since we've removed all events, this should
// do nothing
myObject.trigger('*');
Everything else went well. I'm unable to get "arguments" while implementing myObject.trigger("*"); unable to read arguments object / parameters while implementing "*" and hence throw undefined.
My JSFiddle
Disclaimer
I obviously dont know what school you go to or anything, but please don't fool yourself trying to fool your teachers. With a few simple questions they'll know if you understand the material or not, and if you show up with a good answer but no knowledge to back it up, they will know what's up. I'm not accusing you of this, just a friendly word of advice of someone who has had good connections with his teachers after graduating last year ;)
So, how do we do this? Basically, you will have to add some functionality to the prototype of object, at least if you want this to affect all objects made afterwards. You can always create your own class and add the function to that prototype if you only want that class to have this functionality.
We need 3 functions added to the prototype, on, off and trigger of course. On top of that we add one extra property called events, initially an empty object.
You can look at the raw code for all these in the jsfiddle, I will only go through the structure and logic of the code here.
events will hold all the handlers (functions) associated with each event. When adding an event for the first time, we add a eventName property to the events object, the value for this property is initially an empty array.
on will find (or create) the array linked to eventName in events, and push the function into the array (note we do not call the function at this time, we simply store the reference to the function in the array).
off will iterate the array of eventName, and if it finds the same function (note the ===), remove it from the array.
trigger will iterate the array of eventName and call each function. Note that the function is called with the this keyword in the function set to the object, and with the same parameters as the trigger function was called (except eventName, the first parameter, which is filtered out). Yes that means you can pass as many parameters as you want to trigger(), and they will all be passed to each handler.
I won't go into detail what things like splice, slice, ===, arguments and apply do exactly, I'm sure you can find more and better information about that elsewhere on the world wide interwebs.
There's a lot more you can do for this, like making the events object invisible through some nice uses of scoping, but that wasn't part of the question so I didn't bother with that.
If you have any more questions after looking through this, feel free to ask. I also didn't test it extensively so if you find any bugs, let me know.
EDIT: I didn't read through the comments at first, but I now also added support for the '*' wildcard. Basically the functions now check for the wildcard and will iterate all eventNames on the event object when removing or triggering. You can also remove all functions for an event by not giving a function or by giving the same wildcard, but with an eventName.
EDIT2: had some bugs running the teacher's code, realized I forgot to check for hasOwnProperty while iterating. Look that one up, it's very important when working with prototypes!
I now put in the teacher's code in my jsfiddle, to show you that it works :)
jsfiddle with own code
jsfiddle with teacher code
EDIT3 - about the 'undefined' log.
The teacher's code calls .trigger 5 times, and you should see 4 console logs and as far as I can tell, they are all correct.Let me run through each trigger, and the subsequent console logs.
You add a handler to myEvent, which logs the first parameter
You trigger myEvent, with parameter => The parameter (the object), is
logged.
You add a handler to yourEvent, which logs a hardcoded
string.
You trigger yourEvent, no parameter => The hardcoded string is logged'
You trigger * with no parameter, all handlers run => undefined is logged, since no parameters were given, data in myEvent's handler is undefined. The hardcoded string is also logged
You remove the myEvent handler, trigger myEvent and confirm no functions are called
You remove all event handlers, trigger * and confirm no functions are called from any events.
I honestly don't know what you expected to happen on step 5, since you give no parameter, the data is assigned undefined, that's intended behaviour.
If you want to merge the data given in step 2 so it remains on the object, then instruct so in your handler. (for example, iterate all properties of data and add them to this, then log this). Right now you simply pass it data, it gets logged, and then thrown away. You can also add a parameter in step 5, and then all handlers will receive it (including the yourEvent handlers, but that one doesn't assign nor use it).
document.getElementById("myBtn").addEventListener("click", displayDate);

Is there actually a good reason for jQuery to manipulate 'this' keyword in event handlers?

Given the following, common scenario:
console.log(this); // window or any parent object
$('.selector').on('click', function(event) {
console.log(this); // clicked DOM element
});
var myFunc = function() {
console.log(this); // window or parent object
}
Since version 1.3 jQuery adds the event.currentTarget when binding event handlers for which counts event.currentTarget === this, so is there actually a good reason to manipulate this and switch context? Doesn't this behaviour generally go against the unspoken rule of "don't change keyword values" (like undefined = 'not defined')?
This "feature" of jQuery makes a lot of OOP less efficient and awkward imho, when we need to either cache the original this in a variable like self or use helpers like jQuery.proxy to reassign context to event handlers.
My question: is this just a relic of early jQuery implementations kept alive or is there an actual benefit which I cannot see (except maybe the slightly more convenient way than accessing event.currentTarget to get the element...)?
Let's say you've got an object with some methods on it:
var object = {
click: function() {
alert(this.property);
},
property: "Hello World"
}
You can call object.click() and, as you'd expect, you'll get "Hello World" in the alert.
You'd like to be able to use that "click" function as an event handler:
$("button").on("click", object.click);
However you discover that that doesn't work, because jQuery invokes the "click" function with this set to the DOM node for the clicked button. This is irritating. It's also inevitable because of the semantics of JavaScript function calls.
When you call the "click" function by way of a property reference, the language arranges for this to refer to that object. That's why object.click() works. However, when you fetch the reference to the function and pass it across a function boundary (as in the call to .on()), that relationship is lost. All that the jQuery method gets is a plain unadorned function that has absolutely no inherent relationship to the original object involved in its definition.
Thus, jQuery really has only two choices. The first is that it could make explicit the fact that the function is unconnected by arranging for this to be undefined. That wouldn't be very useful however. The other choice is to pick something interesting for this, and that's what the library does. Note that the native DOM level 0 event dispatch mechanism does the same thing.
The reason is that jQuery wants to mimic how regular event handlers (ones created without jQuery or any other library) works. In regular event handlers the value of this refers to the DOM node that triggers the event if there is one.
One could in fact consider that this is an example of jQuery not manipulating built-in behavior.

How to bind to all custom events in jQuery

I know it's not possible to bind to all DOM events and I know you can bind to multiple events by supplying a space-separated list.
But is it possible to bind to all custom events (preferably filtered by a wildcard pattern like 'abc*' or name-space)?
Edit:
To clarify, I have created some custom widgets that respond to some custom events. For example, they all handle an event called stepReset and resets their internal models.
After I've written these, I realized events don't bubble down, so the call $(body).trigger('stepReset') basically does nothing. As a result, I am considering adding an umbrella event handler on all widgets' parent elements to propagate all relevant events down.
(I know this is not an elegant solution, but I forgot to tag elements with handlers with a common class, so there's no easy way to use select them all.)
With regards to your upcoming edit, you can retrieve all bound events by accessing the object's data:
var boundEvents = $.data(document, 'events');
From here, you can iterate over the resulting object and check each property for your chosen wildcard character, or iterate over that property's array elements and check the namespace property of each.
For instance,
$.each(boundEvents, function () {
if (this.indexOf("*")) // Checks each event name for an asterisk *
alert(this);
// alerts the namespace of the first handler bound to this event name
alert(this[0].namespace);
});
If I understood you correctly, you can iterate over the special events object to get a list of custom events (including those specified in the jQuery source code). Here's an ES5 example, you will need to adapt it yourself for older browsers or use a polyfill for Object.keys:
var evts = Object.keys(jQuery.event.special).join(" ");
$("#myDiv").on(evts, function (e) {
// your code here
});

global custom events in jQuery

I want to use custom jQuery events independent of DOM elements, but I'm not sure what the best way is to achieve this.
Here's what I started out with:
// some system component registers an event handler
$().bind("foo.bar", handler); // foo is my app's namespace, bar is event name
// another part of the system fires off the event
$().trigger("foo.bar", { context: "lorem ipsum" });
After looking at jQuery's source, in particular its handling of global AJAX events, I figured this should work:
$.fn.bind("foo.bar", handler);
// ...
$.event.trigger("foo.bar", { context: "lorem ipsum" });
However, it appears that my handler function is never even called.
Am I perhaps going about this the wrong way?
If you're using jQuery >1.4 then $() returns an empty jQuery collection which would mean that no event handler is actually bound to anything. Before 1.4 it would have returned the same as jQuery(document).
It might be better to simply have a global namespace (an actual object) and then add events to that:
var FOO = {};
$(FOO).bind("foo.bar", handler);
$(FOO).trigger("foo.bar", { context: "lorem ipsum" });
I found my way here because I was looking to implement the publisher/subscriber pattern using namespaced custom events using jQuery. While the accepted solution is a way to use $.event.trigger() in a way that is not tied to DOM elements, it won't work well for a true global event implementation in a publisher/subscriber architecture (such as with a complex UI with many asynchronous actions), where you want to have arbitrary objects/elements listen for a custom event.
Through experimentation, I've found that the real answer to why AnC's events were not firing is because jQuery apparently doesn't allow the "." (period) character in custom event names...but underscores seem to be ok.
So, if you name your events something like foo_bar (rather than foo.bar), your code should work as expected. Tested with jQuery 1.4.4.
Edit: Just to be clear - I mean that periods aren't allowed for custom events if you want to use the $.event.trigger() mechanism. In scenarios where events are being triggered by objects or elements, periods seem to be ok...not sure if this is a bug or by design.

Are jQuery events automatically unbound upon destruction of what they where bound to?

If I have the following code in two functions of an object:
add: function()
{
// create trip.
var trip = new Trip();
// add the trip using its id.
this.trips[trip.id] = trip;
},
remove: function(tripId)
{
// remove trip.
delete this.trips[tripId];
}
NOTE: The constructor for the Trip object binds a bunch of custom jQuery event handlers to itself.
Will the event handlers bound to the Trip object be automatically destroyed/cleaned up when the Trip object is deleted?
Would the same occur for a DOM node if it was removed and had event handlers bound to it?
Also I read that objects are not cleaned up by the garbage collector until all references to them no longer exist, so do the event handlers bound to the object by itself count as references and prevent the object from being cleaned up, even when I am no longer referencing it?
The event will not be deleted as jQuery maintains a central repository of all bound event handlers, and does know if or when you deleted an associated object using delete. Try this little test to confirm. (jQuery 1.4.2 only)
jsfiddle link:
// 1. a regular JS object
var root = {};
// Since we can't delete anything created with var (except on Chrome),
// we use an object property here. An array value works just as well,
// which is already the case in your example.
root.o = {};
// 2. at this point, jQuery creates an internal property
// jQuery<UNIQ_ID>, for example jQuery1277242840125 inside object o
$(root.o).bind("myEvent", function() { alert("here"); });
// 3. get the *internal* index jQuery assigned this object:
// there's only 1 property, so we just enumerate and fetch it.
var internalIndex;
for(var prop in root.o) {
internalIndex = root.o[prop];
}
// 4. delete the object
delete root.o;
// 5. query jQuery internal cache with the internal index from step 3
console.log(jQuery.cache[internalIndex].events);
​
Step 5 should log an array of all event types that were associated with the ex-o object, including "myEvent", and it's associated handler, so no the bound events handlers will not delete automatically. Here's what I see get logged (stripped out irrelevant properties):
▾ Object
▾ myEvent: Array (1)
▾ 0: Object
▸ handler: function () { alert("here"); }
namespace: ""
type: "myEvent"
length: 1
The object deletion, however, is not affected, and that will be deleted as expected. However, it is a hole in the wall kind of a situation since there is associated data somewhere in jQuery's cache that will remain there.
It seems that although you can bind events to plain JavaScript objects, you cannot unbind them. It appears jQuery assumes the object is a DOM node when unbinding, and throws the following error:
Uncaught TypeError: Object #<an Object> has no method 'removeEventListener'
Even the part about being able to bind events to objects is, I believe, undocumented. So you have to be a little careful on this, as the following will not work when trying to clean up the event handler references for that object:
$(object).remove()
$(object).unbind(..)
As a workaround, when cleaning up the Trip object, you can explicitly call removeData to do the job.
$(object).removeData();
As I've already mentioned, it's getting knee-deep with jQuery's internals, so you might want to look at an alternative solution, or be wary that library upgrades can easily break your code, which is not very unlikely.
As far as I know, you can only bind event handlers to nodes, or, in special cases, the window, document, etc. For DOM nodes, the event handlers will be removed. Even if they weren't, they wouldn't be able to be triggered anyway. Deleting the object will remove the event handlers associated with it. The event handlers should not prevent the object from being garbage collected.
Would the same occur for a dom node if
it was removed and had event handlers
bound to it?
this.handlerClick = function () { ... };
$(this.testDomNode).bind('click', this.handlerClick);
this.testDomNode.parentNode.removeChild(this.testDomNode);
Using the above code and testing with FireQuery in FireFox removing the dom node does not unbind the handler from the event,
it seems you have to explicitly unbind the handler before removing the dom node as follows:
$(this.testDomNode).unbind('click', this.handlerClick);
this.testDomNode.parentNode.removeChild(this.testDomNode);

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